1. Field of the Invention
The present invention relates to a method of purification by extraction of at least one substance to be isolated from a liquid medium.
2. The Prior Art
Many of the industrial methods of purification now practised are included in the following list, with their disadvantages in parentheses: salting-out (distillation of the agent modifying the solubility), precipitation (acid or base from displacement of the precipitating agent and distillation sometimes azeotropic), liquid/liquid extraction (sometimes with two expensive solvents, with regeneration and progressive loss of efficiency of the solvents), ionic flotation (long and costly), ion-exchange resins (difficult to apply in acid media) and electrolysis (not very efficient, high energy consumption).
In the special case of the production of pure or extra-pure phosphoric acid, if synthesis is excluded, this is now made principally by two methods: either by the "dry" method, with high energy consumption, using "ore-coke-silica" furnaces at more than 1000.degree. C. followed by bubbling through H.sub.2 O of the P.sub.2 O.sub.5 extracted from it; or by the "wet" method using attack by an acid, mainly sulphuric acid (whose evolution of heat is quite considerable), but also nitric acid (corrosive with release of noxious NOx), hydrochloric acid (very corrosive), or perchloric acid (danger of explosion), on phosphate ores given prior treatment (by calcination, crushing, screening) yielding a crude phosphoric acid solution. The Ca salts formed following the acid attack are removed by crystallisation of the sulphates (from 80.degree. C. to 110.degree. C.) or the nitrates (at a temperature &lt;-5.degree. C. and with a low filterability) or again by carrying out a liquid/liquid extraction of the phosphoric acid, leaving the calcium chlorides untouched. The crude acid solutions are then concentrated by evaporation (noxious vapours) up to a 40% or 50% P.sub.2 O.sub.5 content (crude solution, sometimes called "green" or "black" acid in the case of attack using sulphuric acid).
The wet method using sulphuric acid, which covers the majority of the world tonnage of economically valuable phosphates, then directs the green acid either to the production of phosphate salts (for example, of superphosphates and nitrogenous phosphates for fertilisers) or to purification leading to pure or extra-pure phosphoric acid.
The aim of the method according to the invention is to overcome the disadvantages, indicated in parentheses, of the aforesaid present industrial methods of purification.
Methods are also known for the purification of aqueous acid solutions, such as crude phosphoric acid solutions, in which a substance is introduced which, with the acid, forms a precipitate in the form of a phosphate. Known examples are the addition of urea to obtain urea phosphate crystals (Chemical Abstracts 84:152820 of DE-A-2511345), that of melamine to form melamine phosphate (Chemical Abstracts 10: 138052) and that of 1,4-dioxan to form an addition product (salt) of this substance and H.sub.3 PO.sub.4 (Chemical Abstracts of the Japanese patent application Kokai JP 51143597).
These methods turn out to be relatively complicated when it comes to separating the phosphoric acid from the salt or from the adduct formed. Use is generally made of nitric acid to displace the phosphoric acid and to form a nitrate of the precipitating substance. In other cases, through elution using an aqueous ammonia solution, ammonium phosphate is produced, isolating the substance used for the precipitation and thus enabling it to be recycled.
A Moroccan patent no 23439 is also known, in which the phosphoric acid is subjected to the action of an MSA compound of unspecified composition which puts the phosphoric acid into a form which can be purified by simple filtration. The phosphoric acid and the MSA compound are then recovered separately in regeneration reactors. The regeneration step is not indicated in this document either.
In Cellulose, Lignin and Paper-23, 1957, column 18606, a description is given of the precipitation, using aniline, of aniline-H.sub.3 PO.sub.4 from a solution containing dissolved phosphates. Once again this adduct is treated by NH.sub.3 in order to produce ammonium phosphate and the aniline can be extracted and purified from the reaction medium only after several operations, including a purification with Ca(OH).sub.2 and an extraction with benzene.
In Chemical Abstracts, vol 60, column 6515, a description is given of a method for recovering phosphoric acid from a solution containing phosphoric acid or phosphates by treatment with an aromatic amine, particularly aniline or p-toluidine, by heating to 102.degree. C. This gives an amine phosphate of high purity. However, this reference does not explain how to recover pure phosphoric acid from the amine phosphate.
All these methods describe an operation which is generally well known, that of a joint precipitation of the substance added to the liquid medium to be treated and of a substance which occurs in the treated liquid medium. These methods are then used to produce a derivative (a salt, for example) of the acid contained in the liquid medium or of the precipitating agent, without managing to produce this substance (the acid) in the pure state, and simultaneously to recover the precipitating agent in a recyclable manner. The formation of new salts necessitates the introduction of new substances, such as HNO.sub.3 or NH.sub.3, into the liquid medium.
The aim of the present invention is to solve the problems of cost, pollution and energy consumption associated with most of the methods for separating substances from a liquid medium.